Ligands interacting with molecules of the TNF (tumor necrosis factor) receptor superfamily have pivotal roles in the organization and function of the immune system. While regulating normal functions such as immune responses, hematopoiesis and morphogenesis, the TNF family ligands (also called cytokines) play a role in tumorgenesis, transplant rejection, septic shock, viral replication, bone resorption, rheumatoid arthritis and diabetes (Aggarwal, 2003). The TNF ligand family comprises 18 genes encoding 19 type II (i.e. intracellular N terminus and extracellular C-terminus) transmembrane proteins, characterized by the presence of a conserved C-terminal domain coined the ‘TNF homology domain’ (THD). This domain is responsible for receptor binding and is thus critical for the biological activity of the TNF ligand family members. The sequence identity between family members is ˜20-30% (Bodmer, 2002). Members of the TNF ligand family exert their biological function as self-assembling, noncovalent trimers (Banner et al, Cell 1993, 73, 431-445). Thus, the TNF family ligands form a trimer that is able to bind to and to activate the corresponding receptors of TNFR superfamily.
4-1BB (CD137), a member of the TNF receptor superfamily, has been first identified as a molecule whose expression is induced by T-cell activation (Kwon and Weissman, 1989). Subsequent studies demonstrated expression of 4-1BB in T- and B-lymphocytes (Snell et al., 2011; Zhang et al., 2010), NK-cells (Lin et al., 2008), NKT-cells (Kim et al., 2008), monocytes (Kienzle and von Kempis, 2000; Schwarz et al., 1995), neutrophils (Heinisch et al., 2000), mast (Nishimoto et al., 2005) and dendritic cells as well as cells of non-hematopoietic origin such as endothelial and smooth muscle cells (Broll et al., 2001; Olofsson et al., 2008). Expression of 4-1BB in different cell types is mostly inducible and driven by various stimulatory signals, such as T-cell receptor (TCR) or B-cell receptor triggering, as well as signaling induced through co-stimulatory molecules or receptors of pro-inflammatory cytokines (Diehl et al., 2002; von Kempis et al., 1997; Zhang et al., 2010).
Expression of 4-1BB ligand (4-1BBL or CD137L) is more restricted and is observed on professional antigen presenting cells (APC) such as B-cells, dendritic cells (DCs) and macrophages. Inducible expression of 4-1BBL is characteristic for T-cells, including both αβ and γδ T-cell subsets, and endothelial cells (reviewed in Shao and Schwarz, 2011).
CD137 signaling is known to stimulate IFNγ secretion and proliferation of NK cells (Buechele et al., 2012; Lin et al., 2008; Melero et al., 1998) as well as to promote DC activation as indicated by their increased survival and capacity to secret cytokines and upregulate co-stimulatory molecules (Choi et al., 2009; Futagawa et al., 2002; Wilcox et al., 2002). However, CD137 is best characterized as a co-stimulatory molecule which modulates TCR-induced activation in both the CD4+ and CD8+ subsets of T-cells. In combination with TCR triggering, agonistic 4-1BB-specific antibodies enhance proliferation of T-cells, stimulate lymphokine secretion and decrease sensitivity of T-lymphocytes to activation-induced cells death (reviewed in (reviewed in Snell et al., 2011).
In line with these co-stimulatory effects of 4-1BB antibodies on T-cells in vitro, their administration to tumor bearing mice leads to potent anti-tumor effects in many experimental tumor models (Melero et al., 1997; Narazaki et al., 2010). However, 4-1BB usually exhibits its potency as an anti-tumor agent only when administered in combination with other immunomodulatory compounds (Curran et al., 2011; Guo et al., 2013; Morales-Kastresana et al., 2013; Teng et al., 2009; Wei et al., 2013), chemotherapeutic reagents (Ju et al., 2008; Kim et al., 2009), tumor-specific vaccination (Cuadros et al., 2005; Lee et al., 2011) or radiotherapy (Shi and Siemann, 2006). In vivo depletion experiments demonstrated that CD8+ T-cells play the most critical role in anti-tumoral effect of 4-1BB-specific antibodies. However, depending on the tumor model or combination therapy, which includes anti-4-1BB, contributions of other types of cells such as DCs, NK-cells or CD4+ T-cells have been reported (Melero et al., 1997; Murillo et al., 2009; Narazaki et al., 2010; Stagg et al., 2011).
In addition to their direct effects on different lymphocyte subsets, 4-1BB agonists can also induce infiltration and retention of activated T-cells in the tumor through 4-1BB-mediated upregulation of intercellular adhesion molecule 1 (ICAM1) and vascular cell adhesion molecule 1 (VCAM1) on tumor vascular endothelium (Palazon et al., 2011).
4-1BB triggering may also reverse the state of T-cell anergy induced by exposure to soluble antigen that may contribute to disruption of immunological tolerance in the tumor micro-environment or during chronic infections (Wilcox et al., 2004).
It appears that the immunomodulatory properties of 4-1BB agonistic antibodies in vivo require the presence of the wild type Fc-portion on the antibody molecule thereby implicating Fc-receptor binding as an important event required for the pharmacological activity of such reagents as has been described for agonistic antibodies specific to other apoptosis-inducing or immunomodulatory members of the TNFR-superfamily (Li and Ravetch, 2011; Teng et al., 2009). However, systemic administration of 4-1BB-specific agonistic antibodies with the functionally active Fc domain also induces expansion of CD8+ T-cells associated with liver toxicity (Dubrot et al., 2010) that is diminished or significantly ameliorated in the absence of functional Fc-receptors in mice. In human clinical trials (ClinicalTrials.gov, NCT00309023), Fc-competent 4-1BB agonistic antibodies (BMS-663513) administered once every three weeks for 12 weeks induced stabilization of the disease in patients with melanoma, ovarian or renal cell carcinoma. However, the same antibody given in another trial (NCT00612664) caused grade 4 hepatitis leading to termination of the trial (Simeone and Ascierto, 2012).
Collectively, the available pre-clinical and clinical data clearly demonstrate that there is a high clinical need for effective 4-1BB agonists. However, new generation drug candidates should not only effectively engage 4-1BB on the surface of hematopoietic and endothelial cells but also be capable of achieving that through mechanisms other than binding to Fc-receptors in order to avoid uncontrollable side effects. The latter may be accomplished through preferential binding to and oligomerization on tumor-specific or tumor-associated moieties.
Fusion proteins composed of one extracellular domain of a 4-1BB ligand and a single chain antibody fragment (Mueller et al., 2008; Hornig et al., 2012) or a single 4-1BB ligand fused to the C-terminus of a heavy chain (Zhang et al, 2007) have been made. WO 2010/010051 discloses the generation of fusion proteins that consist of three TNF ligand ectodomains linked to each other and fused to an antibody part.
However, there is still a need of new antigen binding molecules that combine a moiety capable of preferred binding to tumor-specific or tumor-associated targets with a moiety capable of forming a costimulatory TNF ligand trimer and that have sufficient stability to be pharmaceutically useful. The antigen binding molecules of the present invention comprise both and surprisingly they provide a trimeric and thus biologically active TNF ligand, although one of the trimerizing TNF ligand ectodomains is located on another polypeptide than the other two TNF ligand ectodomains of the molecule.